1. Field of the Invention
The inventive concept disclosed relates generally to methods employed to prevent and/or minimize fuel ignition, fire, and/or explosion in the interior of aircraft fuel tanks. In particular, the inventive concept disclosed is concerned with specific methods of installing shaped, reticulated sheets or blocks, of foam to fill the internal space of fuel tanks of transport category aircraft.
2. Background
Since 1959, there have been sixteen documented incidents of fuel tank ignition events in aircraft. These fuel tank ignition events have resulted in 542 fatalities, 11 hull losses and 3 incidents causing substantial damage. The causes of the fuel tank ignition events were attributed as follows: 4 were caused by external wing fires, 4 by electrostatics, 3 by faulty fuel pumps or wiring, 2 by lightning, and 3 to unknown causes.
On Jul. 17, 1996, TWA Flight 800 sustained an in-flight break-up after taking off from Kennedy International Airport in New York, resulting in 230 fatalities. The National Transportation Safety Board (“NTSB”) conducted a lengthy investigation and determined that ignition of the flammable fuel/air mixture in a center wing fuel tank had occurred, causing an explosion that disintegrated the aircraft in flight. Although the exact ignition source could not be determined, the NTSB concluded that the most likely ignition source was a short circuit outside the center wing fuel tank that allowed excessive voltage to enter the tank through electrical wiring associated with the fuel quantity indication system (FQIS).
The NTSB announced their official findings regarding the TWA 800 accident at a public meeting held on Aug. 22 and 23, 2000 in Washington. D.C. Primarily as a consequence of TWA Flight 800, the Federal Aviation Administration (“FAA”) issued numerous airworthiness directives intended to reduce possible ignition sources and thereby the risk of another fuel tank explosion. On May 7, 2001, the FAA promulgated rulemaking to establish several new transport category airplane fuel tank safety requirements (66 Federal Registry 23086, May 7, 2001). The rulemaking, effective Jun. 6, 2001, included Amendment 21-78, Amendment 25-102 and Special Federal Aviation Regulation (“SFAR”) No. 88 entitled “Transport Airplane Fuel Tank System Design Review. Flammability Reduction and Maintenance Requirements.” SFAR No. 88 required that type certificate holders and supplemental type certificate holders conduct a revalidation of the fuel tank system designs on the existing fleet of transport category airplanes capable of carrying thirty (30) or more passengers or a payload of 7,500 pounds or more.
Legislation was enacted as 14 CFR §25.981 (Rule 25.981) and FAA Advisory Circulars AC 25.981-1B and 25.981-2 were issued to provide compliance guidance. Compliance with Rule 25.981 required each applicant to develop a failure analysis for the fuel tank installation to substantiate that ignition sources would not be present in the fuel tanks. The requirements of this section are in addition to the more general propulsion failure analyses requirements of 14 CFR 25.901 and 14 CFR 25.1309 that have been applied to propulsion installations.
14 CFR §25.981 (a) (3) defines three failure scenarios that must be addressed in order to show compliance with the rule (known as the “three phases” of compliance):                (a) Each single failure, regardless of the probability of occurrence of the failure, must not cause an ignition source;        (b) Each single failure, regardless of the probability of occurrence, in combination with any latent failure condition not shown to be at least extremely remote (i.e., not shown to be extremely remote or extremely improbable), must not cause an ignition source; and        (c) All combinations of failures not shown to be extremely improbable must not cause an ignition source.        
Compliance with 14 CFR §25.981 (Amendment 25-125) requires investigation of the airplane fuel tank system using analytical methodology and documentation currently used by the aviation industry to demonstrate compliance with 14 CFR 25.901 and 25.1309 but with consideration of unique requirements included in this amendment of this paragraph.
The Federal Aviation Administration (FAA) mandates forced certificate holders to develop and implement all design changes required to demonstrate that their aircraft meet the new ignition prevention requirements and to develop fuel tank maintenance and inspection instructions. Specifically, SFAR No. 88 contains six (6) requirements applicable to transport category aircraft: 1) determine the highest temperature allowed before ignition occurs; 2) demonstrate that this temperature is not achieved anywhere on the aircraft where ignition is possible; 3) demonstrate that ignition could not occur as a result of any single point failure; 4) Establish Critical Design Configuration Control Limitations (“CDCCL”), inspections or other procedures to prevent changes to the aircraft that would result in re-introduction or creation of ignition sources; 5) develop visible means to identify critical features of the aircraft where maintenance, repairs or alterations would affect areas or systems of possible ignition; and 6) design of fuel tanks must contain a means to minimize development of flammable vapors in fuel tanks or a means to mitigate the effects of ignition within fuel tanks.
Maintenance of ignition source prevention features is necessary for the continued operational safety of an airplane's fuel tank system. One of the primary functions of the fuel tank system is to deliver fuel in a safe manner. Preventing ignition sources is as important a function of the fuel system as the delivery and gauging of fuel. The failure of any ignition source prevention feature may not immediately result in an ignition event, but a failure warrants maintenance for continued airworthiness because the failure could eventually have a direct adverse effect on operational safety.
There have been various solutions proposed and implemented to comply with the mandated transport category aircraft fuel tank ignition mitigation requirements. Examples of compliance methods implemented include electronic solutions such as the installation of Transient Suppression Devices (“TSD”), Ground Fault Interrupters (“GFI”), and similar current limiting devices. These devices are deficient in that they retain possible failure rates that a “passive.” non-electronic solution could resolve. Clearly, there is a need for a simplified and reliable solution to make implementation of the SFAR No. 88 compliance feasible. A better solution would be a less expensive, passive solution that is applicable to commercial and private transport category aircraft.